The present invention relates to a pipe molding die and a resin pipe molded by the pipe molding die.
A pipe molding die is employed for manufacturing an elongate resin pipe such as a polypropylene pipe and a polyethylene pipe that are used as, e.g., gas pipes.
The pipe molding die is supplied, from a resin extruder, with a molten kneaded resin (the molten kneaded resin will hereinafter be termed a xe2x80x9cmolten resinxe2x80x9d) defined as a raw material for the resin pipe. The supplied molten resin is discharged finally in the form of a resin pipe as an extrusion molded product from the die via a flow path within the die. According to procedures thereof, the molten resin supplied into the die is temporarily expanded in a cylindrical shape, and thereafter gradually throttled down into a pipe in the end that has a diameter corresponding to an application.
In general, the pipe molding die is basically constructed of a die part, a throttle part and a land part. The die part forms the molten resin supplied in from a resin extruder in a cylindrical shape. The throttle part gives a rectifying effect by throttling the cylindrical molten resin fed in from the die part. And the land part uniforms a flow velocity of the resin. Then, those constructive parts are each concentrically arranged in sequence from an upstream side to a downstream side in a flowing direction of the molten resin.
The resin pipe as an extrusion molded product manufactured by the pipe molding die described above is required to have no ununiformity in terms of wall thickness. Namely, it is required that both of an inner surface configuration and an outer surface configuration of the resin pipe be concentrically complete rounds as viewed in cross-section.
It is because problems as shown in the following items (1)-(3) might arise if the pipe has the ununiformity in wall thickness.
(1) It is undesirable in terms of external appearance.
(2) A core deviation tends to occur when in a butt seam fusion to fuse end surfaces of the pipe by butting them with each other.
(3) Contaminations and flaw on the surface of the pipe are undesirable for joining a Joint to the pipe by fusion, and therefore the pipe surface is required to be cut. In that case, an outer peripheral surface of the pipe is fixed by a jig, and the pipe surface is cut by a cutting tool while moving the cutting tool along the pipe. If ununiformity in wall thickness is large, however, the pipe cannot be uniformly held by the jig because of the ruggedness on t he pipe surface, with the result that there might be a firmly fixed portion and a slackened portion to make the pipe unstable. Further, since a distance between the outer surface of the pipe fixed by the jig and the cutting tool is not uniform, an adhesion is poor, and a complete round cannot be obtained even when cut off. Besides, unevenness in cutting is to appear. A cutting quantity must increase in order to-prevent the unevenness in cutting, and correspondingly extra pipe raw material is needed.
Such being the case, it is a general practice that a flow of the resin extruded from the die is kept constantly in whichever position on a flow path within the die to uniformize the wall thickness of the resin pipe to be molded by the pipe molding die. Methods of enhancing a rectifying effect and a throttle effect are effective in terms of keeping constantly the flow of the resin through the flow path.
For making an attempt to enhance the rectifying effect and the throttle effect as well, the die must be increased in size. When increasing the size of the die, a pressure necessary for flowing the molten resin has to be risen. Furthermore, if the pressure rises, a temperature of the molten resin increases enough to easily deteriorate the resin or to cause an excessive luster on the pipe surface to such an extent as to be visually undesirable, resulting in a devaluation of a commercial product. Then, pressure tightness of the die and of the extruder must be increased.
Moreover, according to the tests by the present inventors, it has proved that the ununiformity in wall thickness is to occur even when making an endeavor to enhance the rectifying effect and the throttle effect in the technologies contrived so far in the case of manufacturing a pipe that is equal to or larger than 8 mm in wall thickness.
It is an object of the present invention to provide a pipe molding die capable of simply preventing an occurrence of ununiformity in wall thickness of a resin pipe irrespective of a degree of desired dimension of the wall thickness of the resin pipe, and also a resin pipe molded by this pipe molding die.
A pipe molding die according to the present invention comprises a throttle part defined as one the constructive parts thereof. This throttle part includes a core, a shell part fitted to the core, and a resin reservoir as a portion of a flow path. The flow path is formed between the core and the shell part. A molten resin as a pipe raw material flows through the flow path.
The resin reservoir is provided in at least one of the core and the shell part and takes a ring-like shape circumscribing a central axis of the pipe molding die with the central axis centered. Further, the resin reservoir assumes a recessed shape in cross-section.
The thus constructed pipe molding die according to th e present invention, a flow of the resin extruded from the die can be uniformized in whichever position on the flow path within the die owing to the resin reservoir, and it is therefore feasible to restrain a momentum of the flow of the molten resin. Consequently, the flow becomes smooth to enhance a rectifying effect. Accordingly, no ununiformity in wall thickness of the resin pipe to be molded can be seen.
Moreover, a capacity of the resin reservoir may be varied corresponding to a dimension of desired wall thickness of the molded resin pipe, i.e., the resin reservoir may be so formed as to decrease the capacity thereof in the case of a thin resin pipe but increase the capacity thereof in the case of a thick resin pipe. A quantity of the molten resin in a longitudinal direction (The longitudinal direction means from an upstream side to a downstream side of the flow path.) at the throttle part is thereby kept constant regardless of a degree of dimension of desired wall thickness of the resin pipe. The keeping constant of the quantity of the molten resin at the throttle part makes it possible to prevent an occurrence of the ununiformity in wall thickness of the resin pipe.
Thus, a size of the resin reservoir provided in the throttle part as one of the constructive parts of the die, is simply set corresponding to the wall thickness of the resin pipe to be molded, whereby the rectifying effect can be enhanced without increasing the size of the die itself.
The flow path described above may include a throttle taking a constricted shape narrower than other parts along this flow path, and this throttle may be formed with the resin reservoir described above.
Moreover, it is desired that the resin reservoir be formed in such a configuration as not to cause a stagnation and a residence (the stagnation and the residence are hereinafter generically termed a xe2x80x9cstagnationxe2x80x9d) in the flow of the molten resin. It is desirable that a recess cross section of the resin reservoir is formed for example in a curved-surface configuration, especially a semi-circular configuration. In that case, it is preferable that a radius of curvature be 10 mm-100 mm, and an angle made by the central axis and each of tangential lines at both ends of a semi-circular arc of the resin reservoir be 15xc2x0-120xc2x0.
Further, it is more desirable that the radius of curvature be 25 mm, and the angle be 75xc2x0-90xc2x0.
The resin pipe according to the present invention is molded by using the pipe molding die as well as being molded of polyolefine as a pipe raw material.
Polyolefine as the pipe raw material is desirably polyethylene.
Furthermore, it is preferable that the resin pipe be manufactured so that an average wall thickness thereof is set to one of values in a range of 5 mm-50 mm, and that a difference between a maximum wall thickness and a minimum wall thickness of the pipe is equal to or smaller than 1.0 mm and, preferably, equal to or smaller than 0.3 mm.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.